Recently, the rapid development of modern wireless communication systems demands ever-greater functionality, higher performance, and lower cost in smaller and lighter formats. As high-order filters may be used to obtain better bandpass performance, which will obviously increase circuit size. Since the dual-mode resonator can be used as a doubly tuned resonant circuit, the number of resonators for a given degree of filter can be reduced to half, resulting in miniaturized filter configuration. Many dual-mode bandpass filters (BPFs) with good performance have been developed, for example, a dual-mode filter loaded with open-circuited stubs or short-circuited stubs. However, further size reduction for the printed circuit board (PCB) technology becomes a practical problem.
The dual-mode filters utilizing the low temperature co-fired ceramic (LTCC) technology based on the lumped element (L and C) or semi-lumped element design have sprung up. However, as the frequency increases, the parasitic effect, coupling effect and values of the lumped elements can not be accurately predicted and controlled, which would be a common problematic issue in precise and wideband filter designs.
In contrast, the transmission-line-based LTCC filters have no such problems of accurately predicting and controlling the parasitic effect, coupling effect and values of the lumped elements except the large circuit size. Up to now, how to construct a dual-mode filter with distributed elements in smaller and lighter format is still a master challenge.